Stereophonic system employing audio matrixing



Jan. 5, 1965 H. BRUNNER STEREOPHONIC SYSTEM EMPLOYING AUDIO MATRIXINGFiled MarOh l5, 1961 2 Sheets-Sheet i Jan. 5, 1965 D. H. BRUNNER3,164,676

sTEREoPx-:ONIC SYSTEM EMPLoYNG AUDIO MATRIXING Filed March 13, 1961 2Sheets-Sheet 2 /0 /2 AMFL /f/IR fa werf PMMA,

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United StatesPatent O 3,164,676 SYSTEM EMPLOYING AUDIO MATRIXING DavidH. Brunner, bAbington Township, Montgomery County, Pa., assigner, bymesne assignments, to Philco Corporation, Phiiadelphia, Pa., acorporation of Delaware Filed Mar. 13, 1961, Ser. No. 95,394

12 Claims. (Cl.l 179-1) STEREOPHONIC It is customary tol refer to thetwo stereophonic program signals as'the A signal and the B signal .andthis vterminology will be employed herein. The

`tgwo channelsystem mentioned above produces excellent It has theadditional advantage stereophonic effects. v that an existing high;fidelity monauralsystem may b e converted to a high fidelitystereophonic system by ex-V actly duplicating all of the parts of themonaural system `except the source of program signals. However Vsuch asystem has several drawbacks. The necessity of having vtwo'high-powered,V high-fidelity amplifiers and two wide 3,154,676Patented Jan. 5, 1965 lCe'Y wave in a wide band speaker unit. Thedifference signal is amplified 'in a circuit which may have a morerestricted bandwidth than the one employed in the sum channel. The-acoustic wave representing the difference signal is radiated by anelectro-acoustic transducer or speaker unit having a differentorientation from the one employed in the sum channel. Again this secondspeaker unit may be a limited range speaker un-it. The present inventionrelies on acoustic matrixing of the sum and difference acoustic wavesignals to produce the desired Y stereophonic effect.V

For a better understanding of the present inventio together with otherand further objects thereof, reference should now be made to thefollowing detailed description which is to be read in conjunction withthe accompanying d-rawings in which: FIG. 1 `is a block diagram ofonepreferred embodi- ,ment of the invention;

FIG. 1A is a schematic diagram of the matrix circuit of FIG. l; Y

j FIG. 2 is a vector diagram which explains the operation of the systemof FIG. l;

. FIG. `3 is a block diagram of another preferred embodiment of thepresent invention;

FIG. 3A is an alternative loud speaker arrangement which may beemployedin the system of FIG. 3; and

i FIG. 4 is a viewpartially intsection ofV an enclosed range speakerunits makes the system expensive. Also the two wide'rangespeaker units4arel not vreadily accommodated in small rooms.

Several i modified stereophonic `reproducing "systems have beenproposedin` an attempt to overcome one or `more of the disadvantages of.the stereophonic reproducspeaker' vassembly which may be employed incertain embodiments ofthe present invention,

Turning nowr to FIG. ,1, the source of two stereo- `phonically relatedprogram signals is represented by ,block 10.' This source may be astereophonic phonograph' pickup, a'stereophonic tape reproducer, astereo- 'phonic radio receiver or the like. VFor convenience, the

program signal a-t the output connection 12 of source 10 will bereferredto as the A program signal While the signal at connection 14 will becalled the B program signal. Both the A and B `program signals ingsystem employing two identical channels. However 4 none of the systemsproposed heretofore have fully met the need for a compact,relatively-low .cost stereophonlc reproduction system which iseither`self` contained or` constitutes an addition to existing monauralequipment.

. i Therefore it is an object of the present invention' to provide` astereophonic reproduction system which p roi vides excellentstereophonic reproduction with .a mmi- 4 mum yof expensive components.

Another object object of the present invention-is tof provide astereophonic reproduction system which does not require physicallyspaced speaker systems.

An additional objectof` the present invention is to provide astereophonic reproduction system in: which the if' apparent spacingbetween speaker systems may be made greater than actual spacing. i

A further object of the present invention is to provide a stereophonicreproduction system in which the spacing between the apparent-or-effective sources of vacoustic waves maybe varied electronically..

Another object of the invention lstocprovide `animproved stereophonicreproduction system which provides excellent stereoph-onic reproductionwithv only one full range iampliiier and speaker )unit and one or "moreamplitiers and speaker units of limited frequency range.

Still another` object of the present invention is toprovide means forconverting existing `monaural Vequipment to a stereophonic reproductionsystem without modiication of 4existing equipment. l

vention are achieved by matrixin'g thetwostereophonic program signals-ata low level to produce electrical sum and difference signals. The',suln'rv signalisv amplified in a `wide band amplier andV converted toAan acoustic l i In general these and otherobjects ofthe vpresent in- A'difference signal (f1-B). -this operation are Well known in the art,but for 4the sake Itions-of audio frequency transformers. Output lead 22of matrix circuit 16 is coupled to the .input `of an amplifier 26. Thebandwidth and powerhandling capabilities of amplifier 26 must meetthestandards established forthe reproducing systemsincegit handles allare'wide range audio frequency signals. For example, in a high-fidelitystereophonic system thej A and B program signals may `from time to timeinclude componentsihaving frequencies in the range from 20 cycles to20,000 cycles. It is to bef'understood that, in general, the A and Bprogram signals will have different amplitudes and different amplitudevversus time waveforms. y l A The A and Bfprogram signals are suppliedto a matrix circuit 16 which combines the Aand B program. signals toproducea sum signal (A+B) and a Matrix circuitsfor performing `ofcompleteness one simple transformer matrix circuit is shown in FIG. 1A.This matrixV circuit comprises two laudio frequencytransformers 18 and20, each having a primary winding and two independent secondarywindings. The A program signal is supplied to the primary winding'o'ftransformer 18 and the B program signal is 4supplied to vthe primarywinding of transformer 20. "One secondary winding" oftransformer 18 andone secondary Y :winding of transformer 20 are connected in series inthe v'proper polarity toprovide on output lead 22 the (A +B) signal.Thexremaining secondaryjwindings are connected in series to provide onoutputv lead r24the (A -B signal. V` It should be understood that inhigh'delity systems it is `usually preferable to substitute phasesplitter ampliiersand resistor-capacitor coupling for the transformers`18 and Z0 because of the` well-known bandwidth limitan d ofthe Aprogram signal components and all of the B program signal components.The output of amplifier 26 is connected to a wide range electroacoustictransducer or` loud speaker system 28 hy way of a pad 29. YPad 29 may bea conventional L-'pad or Tpad.

Gutputconnection 24 of matrix circuit 16 is coupled to the input of asecond 1amplifier 30. Amplitier 30 may have a more restricted Abandwidthand alower' power handling capacity than amplier 26 without reducing theover-all quality of the system performance. As a typical example,ampliiier 26y may have a passband of from 300 to 10,000 cycles'ffAmpliiier 30 provides oppositely phased tsignals at outputs 32 and 34,'respectively. These signals.

may be provided by usual push-pull'circuitry in the output o'f amplifier30. Gutput connection 32 is coupled to a lspeakerunit 36 by way of a pad37 and output connection 34 is coupled to a speaker unit 33 by'way ofpad 39.

. Pads 29, 37 'and 39 represent one conventional way of changing therelative amplitudes ofthe signals supplied to the three speaker yunits28, 36 and. Other-means of signal amplitude control may be substitutedfor padsV 29,

and 39. For example; suitable gain `control circuits may be included inampliersld and 30. The speaker units .36 and 3S are so oriented that allthree speaker units 28, 36 and 38 projectlsound into a common volume of4space centered at O fronrdifterent directions'. It is desirable but notstrictly necessary that the directive axes of all three speakersintersect at 'the same point in the space..

For reasons which will Vappear presently, it isV desirable rthat speaker28 occupy a positionbetween speakers 35 `and 138. AIt V'-is" alsodesirable but not essential thatrthe respectively, Bc, ABL and BR arecorresponding B com be reliected only in the constants K1 and K2. Inthe'vecspacing PL between speakersl 28 and 36 be equal to the.. i

the directiveaxis of'- speaker 2S. It is also "desirableV but4distancePR between the speakers 23 and'SS and fthat the 'angle 6I',between the linerpar'allel to the .directive 'axis 'olf` -spealerrZS andthedirectiveaxis of speaker bevequal. -to the e193, Vthe!anglehetweengthe second line 'parallel tov not essential that thediiference in elevation'of the three fromthe speakers to the listenerV'location O.V

speakers 2R, and 33 be `small compared to the distance.

, `It lwill now Lbe sho'wn `that thelacoustic waves from the threespeaker units 28, 36 and l38 combine atpointOA to i provide a wavewhich'appears to have'an A component "whichoriginat'es' from an apparentsource TLaat a point to the leftiof'speaker unit 2d and a B `componentwhich Aoriginates from apparent sourc'eTRV at a pointl to the.rightofrspeaker unit 2S. y

It will also be shown that-by properly selectingthe orientation ofYspeaker units 3d and 38 and the relative gains of ampliers 26 andi?,theresultantFA and ,B component signals at point O are the A programsignal component and the B program signal (component, respectively.Since the ,subjective feffect experienced'by a listener at point Odepends only on the Vnetacorustic waves present at point O` and not inthe manner in which they are generated, a listener at point()y freceives the impression that he is listening to a two `sterophonicsystem employing two vappropriately spaced wide vrange speaker units.Portions of the following analytical explanationof the operation of thesystem of FIG.y l are Yjpliedby source 10;

Theacoustic'wav'e signals nradiated the three speaker units 28, 36 Vandmay; beufexpressed as follows:

' 'This condition is niet if `fect, iit isfnow-only necessarytoshow thatthe vectors AN tor diagram of FIG. 2 (but not'in the followinganalytical explanation) it is assumed that K1V=K2=l-v Treating the A andB components separatelyefor the moment, the component AF whichA is theresultant A component for the three speaker units at point O takenparallel to the directive axis of speaker uni-t 28, may be expressed.as'follows:

` AFA +I `1A cos @4x54 cos 0R 4) ywhich may be rewritten asi Y l i i .iAF=A(1+K1 cos @L-fKz cosaR) (5) The component AX which is/the vresultantlA com ponent taken at right angles to the directive axisjof speakerunit Z3 may be expressed as follows;`

AX=K2A sin (2L-.tina sin 9g y. (6) Awhich may be rewritten as: :fr a* ii .AX=A(K1 sin @La-Kzlsinag) (7) ik The yectorlsum oi components AF andAxifsthe vector AN, VThe magnitude of the componentsAN for the three4speaker units may be expressed as:

,Y iANl=\/AF2+A`X2? Y (S) substituting Equations `5` and 7 lin Equation?18, it will be Yseen that Y' If the system is totp'roduceatruestereophonicfelf'ect, Ythe amplitude' ratio IAM/{EN} must equalthe.amplitude "A/ B. -Si'nc'e the second-term under the radical'is the samefor |ANI- as for {EN} 7 1' same as wouldk be produced by astereophonic'reproducer- "f havingtwo spacedspeaker units which radiatesolely the f Y l Y K1 CS COS 0R i .y Since vectors AN and BN be madeto'have the necessary aplitudes to produce the desiredstereophoni'crefandyrBN liavel the properfdirect'ion to producethejstere- Y fi Speaker unit 28Sum Signal: A(al-(.130 Y (1.)

f speaker unirse, Left DifriKgAg-BL), (2)

Y ophonic effects. n FIG, 2 that Wanna-:fili

where 6g isfthe angle between the apparent line of direc- VVtionof,theracousticrvector AN ,andthe directive. axis of speaker' .unit as.Equation, 16: e. f-

Substituting. EquationsY *andA v7.. in

' occurs yfor a considerable area around point O.

where B is the angle between the apparent line of direction of theacoustic vector BN and the directive axis of speaker unit 28.Substituting Equations and 11 in Equation 19,

- (K1 sin @L+ K2 sin 0R) Thus the apparent directions of the vectors ANand BN are the same as that requiredfor normal sterophonic effect. t

From Equation 18 or 21 it can be shown that for coincide with the actualsources 36 and 3S. Alternatively, speaker units 36 and 38 may be spacedany convenient `distance apart and the constants K1 and K2 varied byV`changing the relative gainsof ampliers 26 and 30 or the attenuationprovided by pads 37 and 39 until the desired apparent separation isachieved between thev apparent sources TL and TR.

` yIt has been demonstrated experimentally that the acoustic matrixingof the signals from speaker units -corresponding to speaker units 28, 36and `38 ofvl-iIG.- l yIt can also be shown that the acoustic matrixingof the signals from the three speaker units-producesrthe desired stereo`phonic effect for asynunet-rical as well as symmetrical arrangements ofthe speaker units. However asymmetrical placement of the speaker unitsmay require that the constants K1 and K2 have different values.V

It is wellknown that the low yfrequency acoustic wave componentsl oftypical original source material are rela- 6 Y to have a lower powerrating than that of amplifier 26. Speaker units 36 and 38 are notrequired to reproduce low frequency signals. Therefore these speakerunits may be made quite small.

It will be seen that if amplifier 26 and speaker unit 28 comprise theampliiier and speaker unit of a monaural system, no modification ofthese units is required in order to convert this monaural system to astereophonic system of the ltype illustrated in FIG. 1. 'Furthermore thecost of adding amplifier 30, speaker units 36 and 38 and matrix f unit16 will, in general, be far less than the cost of duplieating the wideband amplifier 26 and the wide range speaker unit 28.

The system shown in FIG. 3 is similar to the system shown in FIG. 1except that a single push-pull electrostatic speaker unit 52 has beensubstituted for the -two speaker units 36 and 38 of FIG. 1. Componentsin FIG. 3 corresponding to like components in =FIG. 1 are identified bythe same reference numerals. In the embodiments of FIG. 3, only oneoutput 32 from amplier 30 is required. The push-pull electrostaticspeaker 52 generates the push-pull acoustic waves (A -B) and -(A-B). VInthe embodiment of FIG. 3, the directive axis of electrostatic speakerunit 52 is oriented at right angles to the directive axis of speakerunit 28. The difference acoustic wave (A -B) yfrom the left side ofspeaker uni-t 52 isreected from a suitable reiiecting surface 62 to thelistener location O. Similarly the acoustic wave -(A-B) from the righthand side of -speaker unit 52 is reflected from a second reflectingsurface 64 to the listener locationA O. The reilecting surfaces 612 and64 of FIG. 3 may 'be the walls of fthe room in which the system shown inFIGQS is located. This reection from the surfaces 62 and 64` t producesan apparent source of the (A-B) signal at tively nondirectional and thatthe low'frequency compo- Y nents of the A and iB` program signalstypically have i nearly identical amplitudes. As a result the lowfrequency components .of the difference: signal (A -B will 'haveverysmall amplitudes.` Since the low Vfrequency components contributelittle to theover-all stereophonic effect and since there is a verylittle low frequency energy in the difference signal (A -B), theampliiier 30 may have a lower cutoff frequency of the order of 300cycles without p materially affecting fthe fidelity over-allsystem. f

Since at frequencies above about 10,000 cycles the Wavelength ofacoustic energy is comparable to or less than the inter-ear spacingof'the average listener lthe location of a source emitting only highfrequency signals becomes ambiguous. `For this reason it has been foundthat in many instances components of the difference signal aboveapproximately 10,000 4cycles may be eliminated without adverselyaffecting the over-all performance of the system or lessening of vthesubjective stereophonic'eect. For these reasons, ampliiier 30 may have arestricted bandwidth, for example a lbandwidth of 300 to 10,000 cycles.Since the peak power in most -acoustic Waves occurs in the ofreproduction of the very low frequency signals, the elimination of theloW` frequency signals from amplifier 30 permits amplifier point 52 andan apparent source of the (A-B) signal at point 52". lt will Lbe --seenthat speaker unit 28 and apparent sources SZand 52" correspond tospeaker units 28, 36 and 38, respectively, of FIG. 1. y

`It has been demonstrated that the acoustic waves above 300 cycles aresufiicientlydirective 'so that appreciable acoustic matrixing of the sumand diierence signals radi ated by speaker units 28 and 52 of lFIG. 3occurs at listener location O. The system shown in FIG. 3 has theadvantage .over the embodiment of FIG. 1 that all speaker units may 'beat a single location. For example, the

electrostatic speaker unit 52 may be incorporated in or placed upon thecabinet or enclosure for 'the wide range speaker unit normally employedin a monaural system. One possible arrangement of the speaker unit 52within the same enclosure as speaker unit 28 is shown in` FIG. 4.

Enclosure is provided with ports 82 and`84 for the acoustic waveVdifference signals (A-B) and -(A `-B).` Bales 86.and 88 may .be providedadjacent -ports 82 and 84, respectively, for concentrating the acousticwaves in the desired direction. If enclosure 80 is placed in the Cornerof a room, bailes 86 and 88 may be augmented by or replaced by theadjacent Walls of the room. In setting upV the system of FIG 3, dueconsideration must be given to the reliective properties of the acousticwave reilecting surfacesy 62 and 64.

FIG. 3A shows an alternative speaker arrangement which may be employedin the embodimentofPlG. 3. In FIG. 3A, two electrodynamic' speakers 70and 72 are substituted yfor the electrostatic speaker 52 of FIG. 3.

Speakers 70 and 72 are pointed in opposite directions so that theyproduce approximately the same distributienetV that variousmodiiicationsand other embodiments thereof i l ,c will occur to thoseskilled in the art within the scope of the invention. Accordingly Idesire the `scope of my invention to be limited only by the appendedclair-ms.

l A stereophonicsignal reproducing system comprising system comprisinginput means for providing at first and second outputs electrical signalsrepresentative of the instantaneous sum and the rinstantaneousdifference, respectively, of two' stereophonically related programsigynals, a first 'amplifier `coupled to said first output, a firstelectroacousticwave transducer coupled to the output of said first'amplifier, said first transducer directing acoustic wave energyrepresentative of said instantaneous sum of said two stereophonically-related'program signals in a selectedV path in response to the signalsupplied by said rst amplifier, additional amplifier means coupled tosaid secon-d output, additional electroacoustic wave transducer meanscoupled tothe output of lsaid additional amplifier means, saidadditional ktransducer means producing two differently directed,oppositely phased acoustic waves representative of saidinstantaneousdifierence of said two s tereophonically related programsignals in' response to the Vsignals supplied by said additionalamplifier means, said last-mentioned two acoustic waves being directeddifferently than the acoustic energy lwave produced by'said firsttransducer.

2. A stereophonic signal reproducing system in accordance with'claim 1fwhereinA said first transducer and said aiddirtinal transducer means areso oriented that the paths i of the acoustic wave energy from said firsttransducer ant said oppositely phased waves from said additionaltransducer means are initially directed toward a common location.

3. A fstereophonic signalreproducing system in accord'- ance with claimlwherein said first transducer and said `additional transducer means vareso oriented that the acoustic wave energy from said first transducerand'v said oppositely phased waves from said additional transducermeans'are initially directed along non-intersecting paths. 4.' Astereophonic signal reproducing system in accord'-y ance with claim lwherein said rst transducer and said additional transducer means arelocated at substantially i VVthe same region in space and wherein saidfirst-transducer-V and said additional transducer means are so orientedthat acoustic wavee'n'erg'y from said first transducer and said:oppositely phased waves from said additional transducer means areinitially 'directed along non-intersecting paths. 5. A stereophonicreproducing system comprising a transducer means each adapted to projectacouslic wave energy in ak path centeredaboutfa,separate directive axis,

saidtransducer means directing acoustic wave energy so asito arrive yata common location fromrtdiferent directions in response Vtofelectricalsignals supplied thereto,

a first amplifierfmeans coupling said first output of saidV matrixymeans to saidfirsttransducer, said first transducer producing anacoustic wave'representativefof said instan- 'p taneous sum of said twoprogram signals ink response to the signal supplied `by said Vfirstampliner means, addiional amplifier vmeans coupling said second outputof said matrix means to said second vand third transducers,` said secondand saidlthird transducers producing oppositely phased Vacoustic wavesrepresentative of said instantaneous Vditference of said twoprogram-signals in response to the `signals Supplied by said additionalamplifier means.

6. A stereo honic sicnalre roducinf s stem in accord- P e P s Y l ancewith claim wherein-said first transducer is located intermediate rsaidsecond and third transducers.

V7."A'stereophonic signal reproducing system in accordc ance with claim5 wherein said three transducers are sooriented that the directive axisof said first transducer makes equal angleswith the directive axes ofsaid second and said third transducers.

8. A stereophonic signal reproducing system in accordance with claim 5wherein the directive axes of the three transducers lie substantiaily ina common plane and wherein the directive axis of said firsttransducerbisects the angie formed by the directive axes of said second and thirdtransducers; l l Y v l c 9, A stereophonic signal reproducing systemcomprising input means for providing at iirst and second outputseicctrical signals representative of the instantaneous'sum and theinstantaneous difference, respectively,V of two stereophonically relatedprogram signais, a first electroacoustic wave transducer arranged todirect acoustic wave energy in a selected path, means coupling saidfirst output to said first electro-acoustic wave transducer, said firsttransducer producing `an acoustic wave `representative of saidinstantaneous sum of said two stereophonically related program signalsin response to signals supplied at said i'irst output, additionalelectro-acoustic wave transducer means, additional means coupling saidsecond output tosaid additional electro-acoustic wave transducer means,said additional transducer means producing in response to the signalssupplied at said ysecond output two difierentiy directed, opposielyphased acoustic waves representative of-'said instantaneousdiiierence'of said two program signals, said last-mentioned twoYacoustic waves being directed differently than the acoustic energy waveproduced by said first transducer.

10. A stereophonic signal reproducing system comprising input means forproviding at first and second outputs electrical signals representativeof the instantaneous sum and the instantaneous difference, respectively,of two stereophonically related program signals', a first amplifier'coupled'tosaid first output, a lfirst full range electroacoustic wavetransducer coupled to the output of said first amplifier, said Vfirsttransducer `directing in a `selected path,

stantaneous difference of said -two stereophonically related programsignalsin response ',to the ksignal supplied by said additionalamplifier means, said difieren'tly directed acoustic waves havingafrequency' range substantially less than the full frequency range oisaid two program signalswhich is to be reproducedbyV said system,rsaidlast-mentioned two acoustic waves `being directed differently than theacoustic-energy wave produced by said first transducer.

1l. A stereophonic reproducing system comprising a matrix means adaptedto receive first and second stereo- `phonically related program signals,said matrix means including means for providing at first and-secondVoutputs electrical signals representative of; the instantaneous sum andinstantaneous difference,v respectivel, of said two program signals,`first full range electroacoustic wave transducer means, and second Vandthird limited range electroacoustic 'wave transducer" means each4adapted to project acoustic wave energy ina path centeredabout vaseparate directive axis, saidtransducer means directing acoustic waveenergy so as to' arrive at a common location from different directionsin response to electrical signals supplied thereto, a first amplifiermeans coupling said first output of said matrix means to said firsttransducer, said first transducerproducing in response to the signalsupplied by said first amplifier means an' acoustic wave 'representativeof said instantaneous sum of said two program signals throughoutsubstantially the full frequency range of said two programsignals whichis to be reproduced by said system, additional amplier means couplingsaid second output of said matrix means to said second and thirdtransducers, said second and said third transducers producing inresponse to the signals supplied by said additional amplier meansoppositely phased acoustic waves representative of the instantaneousdifference of said two program signals in a frequency rangesubstantially less than the full frequency range of the two programsignals which is to be reproduced by said system.

12. A stereophonic signal reproducing system comprising input means forproviding at first and second outputs electrical signals representativeof the instantaneous sum and the instantaneous diierence, respectively,of two stereophonically related program signals, a rst full rangeelectroacoustic wave transducer arranged to direct acoustic wave energyin a selected path, means coupling said rst output to said rstelectroacoustic wave transducer, said first transducer producing inresponse to signals supplied by said rst output an acoustic waverepresentative of the instantaneous sum of said two stereophonicallyrelated program signals throughout substantially the full range ofresentative of said instantaneous difference of said two program signalsin a frequency range substantially less than the full frequency range ofthe two program signals which is to be reproduced by said system, saidlast-mentioned two acoustic waves being directed differently than theacoustic energy Wave produced by said first transducer.

Y References Cited in the le of this patent UNITED STATES PATENTS Owenetal Feb. 5, 1963 Olson Sept. 24, 1963 OTHER REFERENCES Crowhurst:Acoustic Matrixing, Audio, November 1960, pp. 19-21, 78-81.

9. A STEREOPHONIC SIGNAL REPRODUCING SYSTEM COMPRISING INPUT MEANS FORPROVIDING AT FIRST AND SECOND OUTPUTS ELECTRICAL SIGNALS REPRESENTATIVEOF THE INSTANTANEOUS SUM AND THE INSTANTANEOUS DIFFERENCE, RESPECTIVELY,OF TWO STEREOPHONICALLY RELATED PROGRAM SIGNALS, A FIRST ELECTROACOUSTICWAVE TRANSDUCER ARRANGED TO DIRECT ACOUSTIC WAVE ENERGY IN A SELECTEDPATH, MEANS COUPLING SAID FIRST OUTPUT TO SAID FIRST ELECTRO-ACOUSTICWAVE TRANSDUCER, SAID FIRST TRANSDUCER PRODUCING AN ACOUSTIC WAVEREPRESENTATIVE OF SAID INSTANTANEOUS SUM OF SAID TWO STEREOPHONICALLYRELATED PROGRAM SIGNALS IN RESPONSE TO SIGNALS SUPPLIED AT SAID FIRSTOUTPUT, ADDITIONAL ELECTRO-ACOUSTIC WAVE TRANSDUCER MEANS, ADDITIONALMEANS COUPLING SAID SECOND OUTPUT TO SAID ADDITIONAL ELECTRO-ACOUSTICWAVE TRANSDUCER MEANS, SAID ADDITIONAL TRANSDUCER MEANS PRODUCING IN